NA TURE 



[July 15, 1922 



qjr is a maximum, 2-42, when the circumference equals 

 the wave-length. 



For clouds of the same mean density J the opacity 

 reaches a sharp maximum when the particles are of 

 this size. At the same time the absorption changes 

 from the non-selective type to the selective type, vary- 

 ing as A -4 . For visual light the maximum opacity 

 occurs when the radius is o-o86/>. A cloud of particles 

 of this size, and of the density of rock (2-7), will exert 

 an absorption of one magnitude if it contains only 1/86 

 of a milligram of matter per square centimetre of cross- 

 section, regardless of its thickness. If the particles 

 are of half this size, or smaller, the selective absorption 

 is almost as complete as for a gas, but may be nearly 

 100 million times as great. 



Obscuration of light in space, therefore, whether 

 general or selective with respect to wave-length, will 

 be produced mainly by dust particles a few millionths 

 of an inch in diameter, unless such particles form a 

 negligible proportion by w-eight of the obscuring 

 cloud. 



It is just these particles, however, which will be most 

 influenced by the pressure of the radiation of the stars. 

 Calculations from more accurate data confirm Schwarz- 

 schild's conclusion that for a particle of the optimum 

 size and the density of water, the repulsive force of the 

 sun's radiation is about ten times the gravitational 

 attraction, and also show that for stars of the same 

 brightness, but other spectral types, the radiation 

 pressure will be about two-thirds as great for ("lass M and 

 increase for the whiter stars, till lor Class B it is fully 

 ten times as great as for solar stars. 



Dwarf stars will scarcely repel dust at all, but giant 

 stars, and especially the very luminous one of Class B, 

 will repel it very powerfully. Only the coarser particles 

 can come near such a star — the finer ones being driven 

 away. This selective removal, from the vicinity of 

 bright stars, of the particles which are most efficient 

 in cloud formation, may explain the fact that the 

 luminous portions of these dark nebulae, though 

 centred upon stars, do not brighten up in their im- 

 mediate neighbourhood so much as might have been 

 anticipated. 



The finest dust must continue to be repelled by the 

 stars, whatever their distance. It may congregate to 

 some degree in interstellar regions, where the repulsive 

 forces from stars on opposite sides are nearly equal, 

 but it can be in no true equilibrium there, and must 

 escape ultimately to an indefinitely great distance. 



Some force, however, operates to hold these dark 

 clouds together, for their outlines are often sharp. This 

 is probably the gravitational attraction of tin cloud 

 itself. 



Taking a spherical cloud as an example we find 

 that, if its mass is 71/ times that of the sun, and its 

 radius R parsecs, the velocity of escape at the surface 

 is 0-092 hfi /?~- km. /sec. The internal velocity of the 

 nebular material is known only in the case of the Orion 

 nebula, where the luminous gas shows irregular varia- 

 tions in radial velocity from point to point, amounting 

 to about 5 km./sec. on each side of the mean. 6 



For a nebula r parsec in diameter (which may be 

 taken as a rough representation of the small black, 

 almost round spot about 15' in diameter, discovered 



' " Publications of the Lick Observatory, Berkeley, Cat," 13, 1918 (98). 

 NO. 2750, VOL. 1 IO] 



by Barnard 7 in Ophiuchus) the mass must be 60 times 

 that of the sun, if the escape velocity is to be 1 km./sec. 



If all this matter were in the form of particles of 

 rock of the optimum size, the extinction for light passing 

 cent rally through the cloud would be 2000 magnitudes. 

 An extinction of 10 magnitudes (quite sufficient for 

 opacity) would be produced if the radii of the particles- 

 wen- 72 /<. 



Though these numerical values are largely con- 

 jectural, it appears probable that the aggregate mass 

 contained in one of these great obscuring clouds must 

 be very considerable — probably sufficient to form 

 hundreds of stars — and that a sensible fraction of the 

 whole mass must be in the form of dust less than o-i 

 mm. in diameter. 



It can easily be shown that any dust cloud which is 

 impervious to light must also be impervious to particles 

 such as those of which it is composed (and to free- 

 moving electrons as well) in the sense that such a 

 particle could not traverse the cloud without a practical 

 certainty of collision. These collisions may account 

 for the existence of dust within the clouds, even if it 

 was not a primitive constituent. 



The transition from these dark nebula? to luminous 

 nebulae in the vicinity of the stars appears to occur in 

 two ways. The first is by simple reflection of the light 

 of the stars : this appears to occur in the nebulosity 

 surrounding the Pleiades, the star p Ophiuchi, and 

 probably in many other cases. The second is by the 

 excitation of gaseous emission, as in the Great Nebula 

 of Orion, which is connected with one of the greatest 

 known regions of obscuration and itself shows signs 

 that obscuring masses lie in front of it. 



Both theoretical considerations, as suggested by the 

 writer 7 and the facts of observation collected by 

 Hubble, 8 indicate that the luminosity of gaseous 

 nebulas is probably due to excitation of the individual 

 atoms by radiations of some sort (aethereal or corpus- 

 cular) emanating from neighbouring stars of very high 

 temperature. In the Orion nebula the stars of the 

 Trapezium (d Orionis) appear to be the source of 

 excitation. 



There is no reason to believe that the luminous gas 

 forms the whole, or even any large part, of the matter 

 present within the region — only that it is selectively 

 sensitive to the incident excitation, and therefore gives 

 out most of the light, just as the gases (carbon com- 

 pounds and nitrogen) do in the coma and tail of a comet. 



If the turbulent motions of the various parts of this 

 nebula are of the same order of magnitude in the other 

 two co-ordinates as in the radial direction, they must 

 correspond to an average proper motion of 1-5 astro- 

 nomical units per year, or about o"-S per century (with 

 Kapteyn's parallax of o"-oc<55). In a million years this 

 would carry a nebulous wisp through 2 , which is more 

 than the whole extent of the nebula. 



It appears probable, therefore, that the aspect of the 

 Orion nebula was entirely different a million years ago 

 from what it is now, as regards its details. There is no 

 reason, however, to suppose that the nebula was not 

 there. We may rather imagine that wisps and clouds 

 of dust, carrying gas with them, are slowly drifting 

 about. Some of "them pass through the field of excita- 



; Russell, H. N., The Observatory, London, 44, 1921 (72). 

 1 lliil, 1,1,, E. P., "Annual Report of the Mount W il>on observatory," 

 1921 ; " Year Book of the Carnegie Institution of Washington," 1921. 



